In its latest travel advisory, the UN agency includes in that category people over 60, and those with conditions such as heart disease, cancer and diabetes.
It also restates its position that blanket travel bans do not prevent the spread of the variant.
South Africa alerted the world about the variant last week.
Many countries then proceeded to impose restrictions on travel from countries in southern Africa.
"Blanket travel bans will not prevent the international spread, and they place a heavy burden on lives and livelihoods," the WHO said in its statement.